The present invention relates to a method of fabricating single-mode optical fiber preforms and a torch for fabricating porous preforms for the core of the single-mode optical fiber preform.
A single-mode optical fiber has an extremely wide transmission bandwidth, and accordingly the single-mode optical fiber is expected to be employed as a high-capacity long distance transmission line in the future. There is known a so-called MCVD (Modified Chemical Vapor-phase Deposition) method which has been used as a method for fabricating a single mode optical fiber preform. In this method, a cladding glass layer and a core glass layer are formed on the inner surface of a supporting silica tube after which the assembly of these layers is collapsed to form an optical fiber preform. The resultant single-mode optical fiber has a small transmission loss. In this respect, the MCVD method is available for the manufacturing of, for example, a single-mode optical fiber with a transmission loss in the order of 1 dB/km or less in the wavelength band of 1.55 .mu.m, which has recently attracted attention. In the MCVD method, however, the length of a single-mode optical fiber obtained from a single optical fiber preform is generally 2 to 5 km and even at most 10 km. Therefore, the MCVD method is disadvantageous for the mass-production of single-mode optical fibers.
Another known method of manufacturing a single-mode optical fiber is the so-called rod-in-tube method. Briefly, in this method, a single-mode optical fiber preform is fabricated first by synthesizing a glass rod to be a core by the so-called plasma method and then by sealing it in a silica tube having proper dimensions. While the rod-in-tube method, when comparing with the MCVD method, is suitable for mass-production of optical fibers, the rod-in-tube method has a disadvantage of the large transmission loss. The large transmission loss in the rod-in-tube method is caused largely by the waveguiding properties of the single-mode optical fiber. In the case of the single-mode optical fiber, a relatively large part of the optical power propagates through not only the core region but also the cladding region. Accordingly, the optical power, through the propagation, is influenced by imperfections and impurities at the boundaries between the glass rod forming the core region and the silica tube forming the cladding region as well as impurities contained in the silica tube, such as OH ions and small bubbles. Because of this influence, it is difficult to reduce the optical transmission loss to a value less than 5 dB/km.
On the other hand, the VAD (vapor-phase axial-deposition) method in which a cylindrical porous preform is first prepared and then is subjected to heating at a high temperature and a vitrifying process to form a transparent preform is suitable for the mass-production of optical fibers. In the VAD method, glass raw material gas such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3, BBr.sub.3 or the like and flame forming gas such as O.sub.2, H.sub.2, Ar, He or the like are led to a glass synthesizing torch. Glass fine particles such as SiO.sub.2, GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3 or the like synthesized by the flame hydrolysis or oxidation reaction of those materials with the glass synthesizing torch are attached and deposited onto a seed rod so as to form a cylindrical porous preform. The cylindrical porous preform thus formed is heated at 1500.degree. to 1700.degree. C. by a high temperature heater and is vitrified into a transparent optical fiber preform.
The glass synthesizing torch is generally formed as a multi-layer tube having such an arrangement that a raw material gas blowing nozzle with a centered circular cross section is coaxially surrounded by an inactive gas blowing nozzle for Ar, He or the like, a combustible gas blowing nozzle for H.sub.2 or the like, and an auxiliary gas blowing nozzle for O.sub.2 or the like, which are disposed in this order. Glass particles, produced by flames blown together with glass raw material gas are sintered and deposited on the seed rod, so that the rod-like glass sintered member is grown in the axial direction. Usually, the synthesizing torch and a flame stream blown out from the torch are disposed coaxially or in parallel with a rotation axis of the seed rod and the porous preform. In forming the porous preform for the optical fiber by the synthesizing torch, the produced glass particles are diffused in a direction orthogonal to the rotation axis, or in the horizontal direction. Therefore, it is difficult to reduce the diameter of the porous glass body thus formed to less than about 40 mm, even if an area of the raw material gas blowing nozzle at the center of the torch is selected as small as possible or the flame stream is converged as intensively as possible.
As an improvement of the VAD method, the synthesizing torch and the flame stream may be inclined by a given angle with respect to the seed rod and the rotation axis of the porous preform. This improved VAD method could stably fabricate the porous preform to a size as small as about 30 mm in diameter. It was, however, difficult to reduce the diameter of the porous preform to less than 30 mm. If the porous preform having a diameter of 30 mm is used as the porous glass body for the core and a cladding layer is deposited on the rod-like porous glass body by using the subsidiary torch, the cladding-to-core diameter ratio is approximately 2 at maximum.
As will be described in detail later, it is required that the cladding-to-core diameter ratio be approximately 3 or more in order to form a single-mode optical fiber. In the above-mentioned example, the ratio is about 2 and the thickness of the cladding layer is insufficient for a ratio of 3 or more. The ratio may be increased by increasing the thickness of the cladding layer. If the thickness is increased in this way so as to obtain a ratio of 3 or more, the diameter of the porous preform for the cladding exceeds 100 mm. The result is that a stress developed therein may crack the porous preform and the excessive diameter renders it inconvenient to handle the porous preform when it is consolidated or vitrified. Because of those disadvantages, it has not been possible to manufacture single-mode optical fibers by taking full advantage of the suitability of the VAD method for the mass-production of optical fibers.